The present invention relates to lithium niobate-based electro-optic devices and, more particularly, to the utilization of a particular device coating to eliminate the need for hermetic packaging of these devices.
Electro-optic devices employing materials such as lithium niobate are often used as modulators in optical communication systems, with optical waveguides formed in a lithium niobate substrate (for example, Ti-diffused waveguides or proton-exchanged waveguides) and electrodes disposed over the waveguides on the top surface of the lithium niobate substrate. In a lithium niobate modulator, by controlling the voltage applied to the surface electrodes, an input optical signal will be modulated as it propagates along the waveguides and thus exit as an optically modulated signal. Other uses of lithium niobate-based devices include attenuators, polarization controllers, switches, sensors, and the like.
While such devices, and in particular modulators, are well-known and used extensively in optical communication systems, the devices are likely to fail within a short period of time if they are not hermetically sealed (i.e., sealed against the ingress of water and/or high humidity levels). Although all of the failure mechanisms are not known, it is suspected that several failure mechanisms occur on the lithium niobate substrate itself. For example, when closely-spaced electrodes (e.g., gold electrodes) have a large voltage difference between them (which is common in most modulator applications), the presence of water or a high humidity level causes the electrodes to short and the device to fail. Corrosion is also an issue when both high electric fields and high humidity are involved.
To overcome these problems, lithium niobate-based devices are conventionally subjected to a hermetic sealing process during packaging to eliminate the presence of moisture in the package and prevent the further ingress of water during the life of the packaged device. Hermetic sealing is expensive and time-consuming, requiring the package to be out-gassed to eliminate moisture, then filled with an inert gas and welded or soldered shut. An enormous savings in cost, as well as an increase in throughput efficiency could be achieved if a practical non-hermetic package could be made, without compromising the performance characteristics required for most commercial applications.
The need remaining in the prior art is addressed by the present invention, which relates to lithium niobate-based electro-optic devices and, more particularly, to the utilization of a particular device coating to eliminate the need for hermetic packaging of these devices.
In accordance with the present invention, a water vapor barrier layer comprising paylene (more specifically, parylene-C, -D, -F or -N) is disposed to coat the lithium niobate-based device. Parylene is very resistant to water penetration, and is effective at preventing localized accumulation of moisture (e.g., H2O or OH) molecules at the electrode interface or on the top surface of the lithium niobate-based itself. The parylene film also provides a barrier to surface migration of metal atoms or complexes from the electrodes. In accordance with the present invention, the parylene is deposited at room temperature. It is desirable to use a coating material with a dielectric constant close to that of air (k=1). Parylene films exhibit a dielectric constant in the range of approximately 2.5 to 5.
It is an aspect of the present invention that the parylene film is highly conformal to device topography and does not add stress to the underlying device. The parylene film also adheres equally well to disparate surfaces, namely, the electrode surface and the top surface of the lithium niobate substrate. With proper pre-cleaning treatment of the lithium niobate substrate surface (e..g, a plasma etching with oxygen or argon, or alternatively, a wet chemical clean), the deposited parylene film will form a high quality adhesive to the underlying substrate and the surface electrodes.
Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.